1. Field of the Invention
The invention relates to a solid-state image sensor and a method of fabricating the same, and more particularly to a solid-state image sensor capable of preventing occurrence of smear, and a method of fabricating the same.
2. Description of the Related Art
FIG. 1 illustrates a unit cell of a CMOS sensor which is an active XY address type solid-state image sensor.
The illustrated unit cell is comprised of a p-type silicon substrate 10, a p-type well 12 formed in the p-type silicon substrate 10, an n-type region 14 formed in the p-type well 12 and acting as a photodiode, a gate insulating film 16 formed on a surface of the p-type silicon substrate 10 and composed of silicon dioxide (SiO2), a reset gate 18 formed on the gate insulating film 16 and composed of polysilicon, a reset drain region 20 composed of an n+ type region, a field oxide film 21 for electrical isolation between regions in each of which a device is to be fabricated, and a light-impermeable film 22 composed of metal and having an opening 23 through which light enters the n-type region 14.
An interlayer insulating film and a wiring layer are formed between the light-impermeable film 22 and the n-type region 14 in which light is converted into electricity.
The n-type region 14 is electrically connected to a source follower amplifier 24. As illustrated in FIG. 1, the source follower amplifier 24 is comprised of (a) a first MOS transistor 26 acting as a selector switch, (b) a second MOS transistor 28 having a source electrically connected to a drain of the first MOS transistor 26, a source electrically connected to a voltage Vdd, and a gate electrically connected to the n-type region 14, (c) a third MOS transistor 29 acting as a load, and having a drain electrically connected to a source of the first MOS transistor 26, and a source electrically connected to a voltage Vss, and (d) an output terminal 30 electrically connected to a source of the first MOS transistor 26 and a drain of the third MOS transistor 29.
The CMOS sensor illustrated in FIG. 1 operates as follows.
First, a high pulse "PHgr"R is applied to the reset gate 18 to thereby cause the n-type region 14 acting as a photodiode, to have a certain potential. Then, a low pulse "PHgr"R is applied to the reset gate 18 to thereby accumulate electric charges in a depletion layer which electric charges have been generated by converting light into electricity.
A potential of the photodiode 14 varies in accordance with the thus accumulated electric charges. Variation in the potential is output through the output terminal 30 of the source follower amplifier 24.
In such a conventional CMOS sensor as illustrated in FIG. 1, since an interlayer insulating film and a plurality of wiring layers are sandwiched between the light-impermeable film 22 and the n-type region or light-electricity converting region 14, the light-impermeable film 22 is much spaced away from the n-type region 14. Hence, light 25 diffracted due to diffraction effect of light having passed through the opening 23 reaches a border area of the light-electricity converting region 14, as illustrated in FIG. 1.
Thus, the conventional CMOS sensor is accompanied with a problem of so-called smear that light reaching a border of the light-electricity converting region or n-type region 14 due to diffraction effect is converted into electricity to thereby generate a false signal.
In addition, the thus generated false signal is trapped in adjacent light-electricity converting regions or in a diffusion layer of the source follower amplifier 24, resulting in another problem that a S/N ratio of image signals is degraded.
In view of the above-mentioned problem, it is an object of the present invention to provide a solid-state image sensor which is capable of preventing generation of smear or the above-mentioned false signal. It is also an object of the present invention to provide a method of fabricating such a solid-state image sensor.
In one aspect of the present invention, there is provided a solid-state image sensor including a first region in which light is converted into electricity, and a second region composed of silicide, the second region at least partially forming a border area of the first region at a surface of the first region.
The second region can interrupt light which would cause smear, from entering the region.
It is preferable that the solid-state image sensor is constituted as a CMOS sensor or a CCD sensor.
It is preferable that the second region is composed of silicide of refractory metal.
There is further provided a solid-state image sensor including a first region in which light is converted into electricity, a reset gate electrode, a reset drain region, and a second region composed of suicide, the second region at least partially forming a border area of the first region at a surface of the first region.
There is still further provided a solid-state image sensor including a first region in which light is converted into electricity, a reset gate electrode, a reset drain region, a second region composed of suicide, the second region at least partially forming a border area of the first region at a surface of the first region, and a third region composed of suicide, the third region covering a surface of the reset drain region therewith.
It is preferable that the second and third regions are formed from a common layer.
There is yet further provided a solid-state image sensor including a first region in which light is converted into electricity, a light-impermeable film having an opening situated above the first region, and a second region composed of silicide, the second region at least partially forming a border area of the first region at a surface of the first region such that the second region interrupts diffracted light coming through the opening, from entering the first region.
There is still yet further provided a solid-state image sensor including a first region in which light is converted into electricity, a light-impermeable film having an opening situated above the first region, a reset gate electrode, a reset drain region, and a second region composed of silicide, the second region at least partially forming a border area of the first region at a surface of the first region such that the second region interrupts diffracted light coming through the opening, from entering the first region.
There is further provided a solid-state image sensor including a first region in which light is converted into electricity, a light-impermeable film having an opening situated above the first region, a reset gate electrode, a reset drain region, a second region composed of silicide, the second region at least partially forming a border area of the first region at a surface of the first region such that the second region interrupts diffracted light coming through the opening, from entering the first region, and a third region composed of silicide, the third region covering a surface of the reset drain region therewith.
In another aspect of the present invention, there is provided a method of fabricating a solid-state image sensor, including the steps of (a) forming a first region in which light is converted into electricity, in a silicon substrate, the first region having an electrical conductivity opposite to an electrical conductivity of the silicon substrate, and (b) forming a second region composed of silicide, the second region forming a border area of the first region at a surface of the first region.
For instance, the step (b) may be carried out by silicifying a border area of the first region or deposition a refractory metal film and heating the refractory metal film.
It is preferable that the method further includes the step of forming a light-impermeable film having an opening situated above the first region.
There is further provided a method of fabricating a solid-state image sensor, including the steps of (a) forming a first region in which light is converted into electricity, in a silicon substrate, the first region having an electrical conductivity opposite to an electrical conductivity of the silicon substrate, (b) forming a reset gate on the silicon substrate, (c) forming a reset drain region in the silicon substrate, the reset drain region having an electrical conductivity opposite to an electrical conductivity of the silicon substrate, and (d) forming a second region composed of suicide, the second region forming a border area of the first region at a surface of the first region.
It is preferable that the method further includes the step of forming a third region on a surface of the reset drain region, the third region being composed of silicide, in which case, it is preferable that the second and third regions are simultaneously formed.
There is still further provided a method of fabricating a solid-state image sensor, including the steps of (a) forming a well in a silicon substrate, (b) forming a first region in which light is converted into electricity, in the well, the first region having an electrical conductivity opposite to an electrical conductivity of the well, (c) forming a reset gate on the well, (d) forming a reset drain region in the well, the reset drain region having an electrical conductivity opposite to an electrical conductivity of the well, and (e) forming a second region composed of silicide, the second region forming a border area of the first region at a surface of the first region.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
In the solid-state image sensor in accordance with the present invention, the second region of silicified region is formed at a border of the first region or light-electricity converting region. As a result, there is no space into which diffracted light which would cause smear is incident. Hence, light which would cause smear is prevented form entering the first region or light-electricity converting region.
Accordingly, it is now possible to prevent generation of a false signal to be generated by light entering a border area of the first region due to diffraction effect. In addition, it is also possible to prevent the thus generated false signal from being trapped in adjacent light-electricity converting regions or in a diffusion layer of a source follower amplifier, and thus, possible to prevent a S/N ratio of image signals from being degraded.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.